Table Of ContentDEEP FLAW DETECTION WITH GIANT MAGNETORESISTIVE (GMR) BASED
SELF-NULLING PROBE
Buzz Wincheski and Min Namkung
NASA Langley Research Center
Hampton, VA 23681
INTRODUCTION
The use of giant magnetoresistive (GMR) sensors for electromagnetic
nondestructive evaluation has grown considerably in the last few years [1-4].
Technological advances in the research and development of giant magnetoresistive
materials has led to commercially available GMR sensors with many qualities well suited
for electromagnetic NDE. Low cost GMR magnetometers are now available which are
highly sensitive to the magnitude of the external magnetic field, have a small package size,
consume little power, and operate at room temperature [5]. Incorporation of these sensors
into electromagnetic NDE probes has widened the application range of the field. In
particular, the low frequency sensitivity of the devices provides a practical means to
perform electromagnetic inspections on thick layered conducting structures.
The incorporation of a commercially available GMR sensor into the Self-Nulling
Probe has been explored in the past [3]. This research showed that the modification could
be performed in a straightforward manner to greatly enhance the low frequency capabilities
of the device. A limiting factor in this previous work appeared to be the increased
background level with decreasing frequency for flaw detection at depths greater than
approximately 5 mm.
In order to improve the signal to noise ratio of the GMR based Self-Nulling Probe
the incorporation of active feedback in the probe design has been investigated. The active
feedback is shown to greatly reduce the background field levels in the interior of the probe
where the GMR sensor is housed. This, combined with image processing to filter out
background variations from the processed data, has resulted in a greatly improved signal to
noise ratio for very deeply buried flaws in conducting materials.
ELECTROMAGNETIC INSPECTION OF THICK ALUMINUM COMPONENTS
Electromagnetic inspection of thick conducting materials requires a low frequency
excitation, due to the skin depth relationship [6,7] which is given in CGS units by
B = B exp(−z/δ), (1)
z 0
δ= c , (2)
2πµωσ
where B is the magnetic field at depth z into the material under test, B is the magnetic
z 0
field at the surface, c is the speed of light, µ is the permeability, ω is the angular
frequency, σ is the conductivity, and δ is the skin depth. The skin depth of a highly
conducting material will be quite small unless the frequency of operation is also lowered.
In order for the skin depth to reach 1 centimeter in an aluminum alloy with µ=1 and σ=1.7
x 1017 sec-1, for example, the operating frequency must be reduced to 130 Hz.
As explained above, an eddy current device designed for deep inspections must be
sensitive to low frequency magnetic fields. Pickup coil type sensors, however, become
ineffective in this frequency range due to Faraday's law of electromagnetic induction which
states
A dB
ε= − ∝ωsinωt. (3)
c dt
The electromotive force, ε, induced around a circuit is proportional to area enclosed, A,
multiplied by the time rate of change of the magnetic field through the circuit [6]. Since
the output of the coil is proportional to the time rate of change of the magnetic field the
effectiveness of the inspection decreases with decreasing frequency.
Giant magneto-resistive sensors respond to the magnitude of the external field
instead of the time rate of change of the field and therefore do not lose sensitivity at low
frequencies. In the absence of an applied field, the resistivity of the GMR element is high
due to scattering between oppositely polarized electrons in the antiferromagnetically
coupled multi-layers of the device. An external field aligns the magnetic moments of the
ferromagnetic layers, eliminating this scattering mechanism and thereby reducing the
resistivity of the material [5]. A schematic diagram of a commercially available GMR
sensor is displayed in figure 1. In this design four GMR elements are arranged in a
Wheatstone bridge configuration on an eight-pin integrated circuit.
Figure 2 shows the calibration data for the GMR sensor displayed schematically in
figure 1 [3]. This data was acquired with the sensor placed in the center of a Helmholtz
pair driven by a precision current source. Fifteen volts were placed across the GMR
bridge, and a 20 dB differential preamplifier was used to amplify the output voltage of the
sensor. Note that the output increases with the magnitude of the magnetic field and that the
sensitivity, the slope of the output voltage versus applied field, drops dramatically in the
low field region between approximately ±0.5 Oe.
GMR Elements
Shield
Flux V Flux
Concentrator Concentrator
Shield GMR Elements
Figure 1. Schematic diagram of commercially available GMR sensor [5].
) 0.25
V
(
ge 0.2
a
t
l
o
V 0.15
t
u
tp 0.1
u
O
r 0.05
o
s
n
e
S 0
-1 -0.5 0 0.5 1
Applied Magnetic Field (Oe)
Figure 2. Calibration data for GMR sensor [3].
GMR BASED SELF-NULLING PROBE WITH ACTIVE FEEDBACK
A schematic diagram of the GMR based Self-Nulling Probe including the active
feedback circuit is shown in figure 3. The design is based upon the NASA LaRC
developed Self-Nulling Eddy Current Probe for surface and near surface flaw detection [8].
The new probe has two main design changes from the original Self-Nulling Eddy Current
Probe. A GMR sensor is used in place of a pickup coil as the field sensor and a second
current source is added to provide active feedback to the GMR sensor location.
The use of active feedback inside the flux-focusing lens enables a complete
cancellation of the sinusoidal stray fields at the GMR sensor location. Previous work has
shown that at low frequencies the magnetic flux leakage around the bottom of the flux
focusing lens results in a large background signal level [3,9]. In the current design the
background signal is removed by applying a feedback signal at the same frequency as the
drive source but 180° out of phase with sensor output to the feedback coil. The feedback
coil is also used to shift the operating point of the GMR sensor out of the low field region
where the sensitivity of the device is low. For this a DC voltage is applied to the feedback
coil to provide a static background field level of approximately 0.75 Oe.
Figure 3. Schematic diagram of GMR based Self-Nulling Probe with active feedback.
Figure 4 displays the drive coil and sensor output waveforms for the GMR based
Self-Nulling Probe at various stages of feedback to the GMR sensor. The data were
acquired at 185 Hz drive with 15 volts to the GMR bridge and 40 dB preamplification. The
waveforms in the figure 4a were taken without any feedback. As was seen in previous
work, the output signal is rectified due to the insensitivity of the GMR sensor to the
direction of the applied field [3]. A significant drop in the output voltage is observed when
the probe is placed on the conducting sample, although a relatively large output voltage is
still observed.
Figure 4b displays the change in the output as the feedback source is adjusted.
With the probe on the sample the application of a DC bias to the feedback coil shifts the
operating point of the sensor away from the zero crossing area and up the calibration curve
to a location of higher sensitivity. This results in an increased output amplitude of the
sensor, as is seen in the right hand plot. Also note that the signal is now at the same
frequency as the drive source, with the rectification of the signal eliminated. The final step
is then to apply a sinusoidal signal to the feedback coil at the drive coil frequency but 180°
out of phase with the probe output. The resulting probe output is a DC shifted amplitude
with only a very small AC component. Adjusting the feedback in this manner cancels the
leakage magnetic fields in the center of the probe and biases the sensor in order to obtain
maximum sensitivity to small changes in the magnetic field caused by deeply buried
defects.
EXPERIMENTAL RESULTS
Once the feedback source is adjusted as described above and illustrated in figure 4b
the sample under test can be scanned. A lock-in amplifier referenced to the drive coil is
used to record both the amplitude and phase of the probe output as a function of position on
the sample surface. The first sample tested was a two-layer aluminum alloy lay-up, as
shown in figure 5. The top layer was formed of an unflawed 4.76 mm thick plate. The
lower layer was formed from a 1 mm thick aluminum plate with fatigue cracks grown from
either side of a drilled center hole. The sample was scanned from the unflawed side with
the probe operating at 375 Hz.
Sensor Output On Sample with DC Bias
Sensor Output in Air
Sensor Output with Active Feedback
Sensor Output On Sample
Voltage Across Drive Coil
2.5 Voltage Across Drive Coil
2 a) No feedback b) Active
feedback
)
V1.5
(
e
g 1
a
t
l
o0.5
V
0
-0.5
0 0.002 0.004 0.006 0.008 0.01 0.002 0.004 0.006 0.008 0.01
Time (Sec) Time (Sec)
Figure 4. Drive coil and probe output waveforms at various stages of active feedback to
the GMR sensor.
Figure 5 displays the geometry and a surface plot of the experimentally measured
probe output amplitude for the first experimental sample. The edge of the bottom plate,
center hole, and fatigue cracks emanating from either side of the hole are all clearly visible
in the surface plot. In addition, a peak in the output amplitude at the location of the fatigue
crack tips is evident in the data. This feature, due to the detailed nature of the perturbed
current path around the flaw, has been extensively studied in the past for surface flaw
detection with the Self-Nulling Probe [10]. The detection of this maximum field amplitude
at the crack tip for deeply buried flaws illustrates the detail that can be obtained with the
GMR based Self-Nulling Probe incorporating active feedback.
A second experimental sample was fabricated to examine the detection capabilities
of the probe for flaws at varying depths in thick layered conductors such as airframe wings.
A set of thirteen, 1 mm thick, aluminum plates with a cross section of 15 x 15 cm2 was
obtained. An EDM notch of 1.4 x 0.0127 cm2 was placed in the center of one of the plates.
The location of this flawed layer within the stack of unflawed plates could then be varied,
and data acquired over the flawed area for each case. Previous results using the GMR Self-
Nulling Probe without the feedback coil showed that the flaw could be clearly detected in
the 6th and 7th layer, but that the signal to noise ratio dropped dramatically for deeper layers
[3].
The capabilities of the modified sensor along with a schematic representation of the
sample are shown in figure 6. The plotted data were acquired at an operating frequency of
185 Hz with the flaw placed in the 10th layer of the sample, beneath 9 mm of unflawed
aluminum. The flaw location is clearly imaged in the experimental data. A double peak
indication is detected since the notch length of 1.4 cm is less than the outside diameter of
the GMR based Self-Nulling Probe (1.9 cm). A peak in the output is observed when the
probe is centered over either tip of the flaw. When the probe is centered over the midpoint
of the notch, however, the majority of the induced current will flow unperturbed around the
flaw tips.
Scan Area
5 cm
10 cm
Amplitude (Vrms)
4.76 mm
1 mm
0.0 0.01 0.02 0.03
Figure 5. Sample geometry and surface plot of experimental data for fatigue crack
detection through 4.76 mm unflawed aluminum alloy plate.
Scan area
Flaw location
Amplitude ( Vrms)
Flaw layer
7.5E-3 8.0E-3
Figure 6. Sample geometry and surface plot of experimental data for 1.4 cm long EDM
notch in 10th of 13 1.0 mm thick aluminum alloy plates.
The results shown above for the 10th layer flaw were achieved by using some
additional data analysis and image processing that were not required for shallower flaws
such as that depicted in figure 5. In figure 5 the amplitude of the probe output voltage was
used to identify the flaw location. In figure 6, however, the amplitude data was combined
with the phase information in order to obtain a phase rotated amplitude. The probe output
voltage was calculated as
V = ACos(ϕ+β) (4)
out
were V is the probe output voltage, A is the root mean square output amplitude, ϕ is the
out
phase of the output waveform with reference to the drive signal, and β is a constant phase
shift applied uniformly to all data points. The phase shift is rotated so as to minimize
output voltage changes due to lift-off effects and maximize the signal due to the deeply
buried flaws. The method is very similar to the standard eddy current practice of rotating
the impedance plane diagram so as to provide a horizontal signal for lift-off variations and
then monitoring the vertical component of the change in impedance.
After the amplitude and phase data were combined a simple image processing
routine was used to enhance the image quality for greater flaw detectability. The image
was first flattened in order to remove any linear drift in the data caused by the presence of a
small angle between the scanning plane and the sample surface or electronics drift during
the scan. A low pass two-dimensional Fourier filter was then applied across the sample set
in order to remove any high frequency noise caused by the scanning system. Figure 7
details the data analysis and image processing steps as applied to the data presented in
figure 6.
A B
Amplitude (mV) Amplitude (mV)
6.5 7.0 7.5 8.0 8.5 6.5 7.0 7.5
C D
Amplitude (mV)
7.4 7.6 7.8 8.0
Figure 7. Data analysis and image processing steps for 1cm deep flaw; (A) sensor output
amplitude, (B) phase rotated amplitude, (C) flattened image, (D) low pass Fourier filtered
data.
SUMMARY
In this paper a design modification to the Very-Low Frequency GMR Based Self-
Nulling Probe has been presented to enable improved signal to noise ratio for deeply buried
flaws. The design change consists of incorporating a feedback coil in the center of the flux
focusing lens. The use of the feedback coil enables cancellation of the leakage fields in the
center of the probe and biasing of the GMR sensor to a location of high magnetic field
sensitivity. The effect of the feedback on the probe output was examined, and
experimental results for deep flaw detection were presented. The experimental results
show that the modified probe is capable of clearly identifying flaws up to 1 cm deep in
aluminum alloy structures.
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